Distortion correction in transmission systems



Dec; 15, 1931. w. P. MAsoN 1,836,809

DISTORTION CORRECTION IN TRANSMISSION SYSTEMS Filed Oct. 3, 1928 Patented Dec. 15, 1931 UNITEDY s'rA'rlss PATENT OFFICE WARREN P. MASON, OF ELAST ORANGE, NEW JERSEY, ASSIGNOR T'O BELL TELEPHONE `LABORATORIES, INCORPORATED, OF YORK, N. Y., A CORPORATION OF NEW e YORK DISTORTION CORRECTION IN TRANSMISSION SYSTEMS Application led October 3, 1928. Serial No. 310,165.

This invention relates to systems for transmitting electrical wave energy, such Vas telephone transmission systems and the like, and particularly to the correction of distortion in such systems. A

From Van ideal qualitystandpoint, a transmission system should be designed so that the currents received thereover are exact copies in every respect ofthe corresponding currents entering the system at the transmitting end; in other words, the Vtransmission system should be distortionle'ss. When electrical waves are transmitted over conductors of relatively great length, the distortion produced in the waves due to the inherent properties of the conductors becomes of increasing importance. In long, high quality systems, such as are now being used'for telephone transmission, program broadcasting, television, etc. the distortion produced by lines is often of such magnitude as to appreciably impair the quality of the received waves,r andy thus reduces the commercial eiiiciency of, such systems.

Distortion in transmission'lines is, in general, of three different kinds; first, attenua- V tion of the transmitted waves Vby the line;`

second, an unequal degree of attenuation in waves of diderent frequencies transmitted over the line; andthird, a relative phase change in the transmitted waves. of such distortion is the improper proportioning of the resistance, theleakance, the capacity, and the inductance of the line.

The Pupin-Campbell andother loading methods have been devised with the object'of reducing attenuation-in the useful frequency range in transmission systems. Such loading systems have been supplemented inY practice by amplifiers, such as the telephone repeater, to compensate for reduction in the amplitude of the transmitted waves. The second type of distortion inherent in transmission systems, an unequal degree of attenuation in the transmitted waves of different frequencies, may be overcome by the use of attenuation equalizing networks at different points in the system, this expedient having been brought to a high state of development. Phase corrective networks or phase compen- The causer sators to supplement other types of'distortion correction, such as loading, have been used in connection with lines to correct phase Ydistortion in the transmitted waves of .a desired range of frequencies. Other networks have been `devised and utilized in connection with transmissiony lines to combine the functions of phase distortion` correction and attenuation equalization in transmitted waves of a desired rangevof frequencies. V The latter networks have been designed, in part at least, by empirical methods, or else in their designall factors contributing to distortion have not been taken into account. Their use, therefore, enables only an approximation of the ideal distortionless line to be obtained.

'An object of theiinvention is to correct distortion in electric wave transmission. p Y

Another and more specific object is to reduce the attenuation and to minimize variations in the velocity of propagation of electric waves over transmission systems.

VIn accordance withv the invention, exact design formulae have been developed for the Vimpedance relations which a structure must satisfy when combined with a transmission line in a given manner in order to completely compensate for all distortion including amplitude and phase distortion produced by the line `on Waves of all frequencies transmitted thereover, and structures have been designed in acco-rdance with these formulae to accomplish the desired result. p A feature of the invention is a unitary structure or network which .when combined with Va transmission line will simultaneously compensate both for amplitude and phase distortion in waves of all frequencies transmitted thereover, and lat thefsame time restore the waves to their original amplitudes.

In accordance with the invention, the compensation for the amplitude and phase distortion produced in waves of all frequencies transmitted over a given length of transmission line is obtained by inserting in the line in the path of the transmitted waves a structure having characteristics equaling in value but effectively of opposite sign to those of the given length of transmission line. y In a preferred embodiment this structure is inserted in series with the given line and comprises an open-circuited section of smooth line half as long as the section to be compensated, and having a characteristic impedance twice that of the section of line to be compensated when the structure is not inserted therein, and in tandem therewith a device which will effectively change the positive impedance of the inserted section of line into a negative impedance of equal value.

In another embodiment the compensation for phase and amplitude distortion for waves of all frequencies transmitted over a section of line, together with a desired degree of am plication, is obtained by inserting in shunt to said given section of line a structure comprising a short-circuited smooth line of the same length as the given section to be compensated, and having a characteristic impedance half that of theV section of line to be compensated when the structure is not inserted therein, and in tandem therewith a device'which will effectively change the positive impedance of the inserted section of line into a negative impedance of equal value.

Substantially distortionless overall transmissionfor wavesof all frequencies over a transmission line of any length may beeffectively obtained by inserting at intervals along the line in the path of the waves a structure such as described above, each structure being designed in accordance with the principles of the invention to completely conipensate for the amplitude and phase distortion in the transmitted waves produced by adjacent portions of the transmission line.

The objects and advantages of the invention will be best understood fromthe following detail description when read in connection with the accompanying drawings, in which Fig.' 1 shows a diagram of a transmission system embodying one form of the invention; Figs. 2 and 3 show diagrammatically different forms of distortion correcting structures which may be used in the system of Fig. l; Fig. 4 shows a diagram of a transmission system embodying a modification of the invention; Figs. 5 and 6 show dierent forms of distortion correcting structures which may be used in the system of Fig. 4; and Fig. 7 shows a type of simulating network which may be used in place of the artificial lines in the structures of Figs. 2, 3, 5 and 6 to approximate thel results obtainable when said artilicial lines are used.

In Fig. 1 is shown a transmission line 1 for transmitting alternating current waves of a plurality of frequencies generated by the source 2 to a load circuit represented by the resistance 3. The line 1 comprises a plurality and a shunt arm of impedance 1" slnh where Zl is the characteristic impedance of the line 1 and F is the propogation constant for a section of the line of length Z.

The method of design of the series impedances ZA is as follows:

The propagation constant P for the compleX line 1 including the smooth sections S1, S2 and the series impedances ZA is given by the equation:

Differentiating this equation with respect to o (where o=21r times the frequency of the transmitted waves) the following equation is obtained: v

The propagation constant P of the line 1 is composed of an attenuation constant A and a phase constant B. Hence,

P dA, .dB g2g-@T95 (3) If equals Zero, then A and B are constants independent of frequency. For this condition, the attenuation distortion in the transmitted waves will be constant, and the velocity of the transmitted waves will be theoretically infinite, since is equal to the delay.

If the left side of Equation (2) is set equal to zero, the followingv differential equation will be obtained:

dl dw The solution of this equation is:

omziem( y dw Y (4) sinh F where C is an arbitrary constant which is independent of the frequency. Substituting this value for ZA in Equation (1) it is found that Y cosh P=0 (6) 1 cosh I sinh P Equation (7) may be physically realized in accordance with the invention by the structure of Fig. 2. The elements of the structure of Fig. 2, which in the system of Fig. 1 would be inserted in the boXes ZA in series with the terminals thereof comprise a section of opencircuited smooth line 4 half as long` as, and with a characteristic' impedance which is twice that. of the section of line 1 to be compensated by the structure, and in tandem therewith, a device 5 which will effectively change the positive impedance of the opencircuited line t to a negative impedance of equal value.

The negativeimpedance Vdevice 5 may be of any suitable type, for example, a feed-back amplifier such as disclosed and claimed in the patent to R. C. Mathes, No. 1,779,382, issued October 21, 1930, which will effectively give between two terminals thereof over a wide range of frequencies, the negative of resistance, inductance, capacity, or complex combination thereof used as the feed-back impedance. If the structure of Fig. 1 of the Mathes patent is used as the device 5 in the structure of Fig. 2 of this application, the terminals ZA of the latter structure would be connected to terminals 16 and 17 of the former structure, and the two terminals of network 4 of the latter structure would be connected to the terminals of boX 14 in the circuit of the former structure so that vnetwork 4 acts as the feed-back impedance Zin said circuit. Y

The open-circuited section of smooth line 4 has beenrepresentedin Fig.' 2by its equivalent T network comprising two series arms each of impedance 2z, umh g and a shunt arm of impedance the elements of these equations having the same significance as in the equations given above.

If a structure such as shown in Fig. 2 is utilized for the series of impedances ZA in the system of Fig. 1, an overall zero attenuation and zero phase shift will be obtained therein for transmitted waves of all frequencies.

If C l, Equation (5) reduces to:

ZAr--ZZZ COthF .of open-circuited smooth line 8 of the same length as, and with a characteristic impedance which is twice that of the section of line 1 to be compensated, and in tandem therewith a device 9, similarto the device 5 of Fig. 2, which will change the positive impedance of the open-circuited line 8 to a negative impedance of equal value.

The branch 7 comprises a section of open circuited line 10, the same length as, and with a characteristic impedance which is 2C times that of the portion of line 1 to be compensated, and in series therewith a shortcircuited section of smooth line 11 half as 'long as, and with a characteristic impedance (2z. mtg

sinh

Each of the series arms of the T network representing artificial line 10 has an impedance Each of the series arms of the T network representing the artificial line 11 has an impedance of 2OZ, tanh g and the shunt arm an impedance of In Fig. 4 is shown a transmission system identical with that of Fig. 1, except that a shunt impedance indicated by the box ZB, is utilized in place of each series impedan-ces ZA of the system of Fig. 1 between adjacent line sections to compensate for amplitude and phase distortion in transmitted waves of all frequencies for each section of the line 1. The method of designing the shunt impedances ZB is as follows:

The propagation constant for the complex line comprising the sections S1, S2, SB, etc. and the shunt impedances ZB is given by the equation:

cosh P -lcosh I -l- 4% sinh I By differentiating Equation (10) with respect to o equating the left side of the differential equation to zero, and solving, the following solution 1s obtained:

ZZ sinh I b- 2 O-cosh 1*/ Substituting this value for ZB in Equation (l0), it is noted that cosh P= (12) Hence, if the structure which will satisfy Equation (11) is physically realizable and is inserted in line 1 of Fig. 4 as the shunt impedances ZB in the manner shown, the complex line can be made to produce an overall uniform attenuation and a theoretically inand the shunt arm thereof has an impedancev finite or very high constant velocity in the transmitted waves of all frequencies.

If C equals 1, Equation (11) reduces to:

Equation (11) may be physically realized in accordance with the invention by a structure such as shown in Fig. 5. The elements of this structure, which in the system of Fig. 5 would be inserted in the boxes ZB in series with the terminals thereof, comprise a shortcircuited section of smooth line 13 half as long as, and with a characteristic impedance half that of the section of line l to be compensated, and in tandem therewith a device 14, similar to the device 5 in Fig. 2, which will effectively change the positive impedance of the short-circuited line 13 to a negative impedance of equal value. The short-circuited length of line in Fig. 5 has been represented by its equivalent T network, each of the two series arms of which have an impedance of Z r -5 mnh-2- us) and the shunt arm of which has an impedance of tanh g 2 sinh 1 f the structure of Fig. 5 is utilized for the shunt impedances ZB in the system of Fig. 4, an overall zero attenuation and a zero phase shift may be obtained therein for transmitted waves of all frequencies.

lf C 1 Equation (11) reduces to:

Z, sinh F Z, I (--y mnh 2 O+1 ZBZQ sinhl)+ t by) (14) the same length as, and with a characteristic impedance which is times that of the section of line 1 to be compensated, and in tandem therewith a device 16, similar to the device 5 in Fig. 2, which will effectively change the positive impedance of open-circuited line 15 to a negative impedance of equal value.

The second of the networks in the structure of Fig. 6 comprises a short-circuited length of smooth line 17 of the same length as, and having acharacteristic impedance which is times that of the section of line 1 to be compensated.

The third branch network in the structure of Fig. 6 comprises a short-circuited length of line 18, half as long as, and with a characteristic impedance which is one-half that of the section of line to be compensated, and in tandem therewith a device 19, similar to the device 16, which will effectively change the positive impedance of the short-circuited line 18 to a negative impedance of equal value.

The open-circuited line 15 is represented in Fig. 6 by its equivalent T network, each of the series arms of which has an impedance of Z, I am mnh r and the shunt arm of which has an impedance of The short-circuited section of line 17 is represented in Fig. 6 by its equivalent vr network, the series arm of which has an impedance of Z1 m Slllll F and each or' the two shunt arms of which has an impedance of Z, I' 2(0+1) 0th The short-circuited line 18 is represented in Fig. 6 by its equivalent T network, each of the two series arms of which has an impedance of Z1 F tanh and the shunt arm of which has an impedance of Z, 2 sinh I If the structure of Fig. 6 is utilized for the shunt elements ZB in the system of Fig. 4f, an overall uniform attenuation and a zero phase shift may be obtained therein for transmitted waves of all frequencies.

A simulating network made up of inductances, resistances and capacities, such as are used. This simulating network is a T" structure, each of the series arms of which comprises a resistance ofvvalueV and an inductance of value in series, and the shunt arm, a capacity of value C and a resistance of value in parallel, where R is the resistance, L the inductance, C the capacity and'G the leakance of the section or' line to be compensated bythe compensating structure.

It should be noted that each impedance element in the structures of the invention which have been illustrated and described contributes both to attenuation distortion correction and phase distortion connection this being an important distinguishing feature vfrom the structures of the prior art in which attenuation correction or phase distortion correction is the sole function of separate elements therein.

- Although the invention has been directed principally to the obtaining or' complete compensation for attenuation and phase distortion, it is apparent that it may be utilized for obtaining any desired amount of attenuation and phase distortion correction.

Although the invention has been described in connection with a particular transmission system comprising a` plurality of uniform smooth sections of line and a plurality of distortion compensating structures spaced at lequal intervals along a transmission line, the

invention is not so limited, but is applicable to a system comprising a smooth or a loaded line in combination with a single distortion compensating structure, or a plurality of distortion compensating structures spaced at unequal intervals along the line, or any other combination within the scope of the appended claims.

What is claimed is:

1. In a system of electrical Wave transmission, a wave conductor, and means for neutralizing the distortion produced in waves of all frequencies transmitted over said conductor, said means comprising an electrical structure inserted in said conductor inv the path of said waves and having the same characteristic impedance as that of the conductor and a propagation constant which is ei'ectively the negative of that of said conductor.

2. In a system of electrical wave transmislGO sion, a transmission line, and means for producing substantially distortionless transmission throughout said line for waves of all frequencies transmitted thereover, said means comprising electrical structures distributed at intervals along said line in the path of said waves, each structure having a characteristic impedance which is equal to that of said line and a propagation constant which is equivalent in value to that of said line and effectively of negative sign.

3. In a system of electrical wave transmission, a transmission line, and means for producing substantially distortionless overall transmission thereover as regards both attenuation and phase for waves of all frequencies, said means comprising an electrical structure inserted in series with said line and traversed by said waves, said structure having a. propagation constant substantially equivalent in Value to that of saidv transmisyI the propagation constant of said line before the insertion of said structure therein.

7. The combination with a line for transmitting waves of a plurality of frequencies,

of means for correcting for the distortion in ,n

phase and amplitude produced in said waves in transmission over said line, said means comprising a structure in series with said line each element of which contributes both to the phase distortion correction and to the -1- -V sion line but effectively of opposite sign, and f a characteristic impedance which is equal to that of said line.

ai.` In combina-tion, a transmission line for transmitting electrical waves varying in lfrequency, and means for compensating for phase and amplitude distortion in waves of all frequencies transmitted over a given portion of said line, said means comprising a structure inserted in said line in the path of said waves, said structure comprising a section of line having characteristics which are equivalent to those of the portion of the line to be compensated, and in tandem therewith a device for effectively changing the positive impedance of said section of line to a negative impedance of equal value.

5. In combination with a transmission line for transmitting electrical waves varying in frequency, means for compensatingl for all distortion in amplitude and phase produced in said waves by a given section of said line, said means comprising a structure inserted in series with said given section of line and traversed by said waves, said structure comprising a network having impedance characteristics equivalent to those of an open circuited smooth line half the length of said given sectionY to be compensated and with a. characteristic impedance twice that of the given section of line to be compensated before said structure was inserted therein, and in tandem therewith a device which will effectively change the positive impedance of the network to a negative impedance of the same value.

6. ricombination in accordance with that of claim l, and in whi-ch the said electrical structure is in series with said conductor and has an impedance which is 2z; 00th g where Z1 is the characteristic impedance and 

